Cement slurries are used in the construction of wells such as oil and gas wells. One such use is in the setting of casing in the well. In this procedure, a casing (typically a steel tubular liner) is positioned in the well and a cement slurry is pumped from the surface down the casing so as to return to the surface in the annulus between the casing and the borehole wall. The cement fills the annulus and, once set, holds the casing in place and provides fluid isolation between the zones through which the borehole passes. In another use, a cement plug can be set in a well to prevent any further production of fluids to the surface.
Many wells can be several thousand metres in depth. This means that the temperatures at the bottom of such wells may be significantly elevated temperatures, sometimes in excess of 250° C. (482° F.). This can lead to problems in effective placement of the cement slurry. The time taken to pump a cement slurry into a deep well can mean that the onset of thickening caused by cement setting can become a problem, potentially leading to setting of the cement before it is properly placed either around the casing or as a plug.
This setting phenomenon has lead to the development of a series of additives for the cement slurry known as ‘retarders’. These additives act on the cement slurry to delay setting for a sufficient period of time to allow the slurry to be properly placed. Examples of retarders for use in borehole cement slurries can be found in U.S. Pat. No. 5,503,671 and U.S. Pat. No. 5,503,672, the disclosures of which are incorporated herein by reference. These retarder systems are based on the use of mixtures of borate salts (e.g. sodium tetraborate decahydrate, boric acid, sodium pentaborate or potassium pentaborate) and ethylenediaminetetra (methylenephosphonic) (EDTMP) acid salts. However, these systems have been found difficult to use at temperatures of 250° C. (482° F.) and above.
This invention seeks to provide a cement retarder system that is effective at high temperatures, particularly when used with high density cement slurries that are commonly used to plug deep, hot wells. The invention is based on the use of a copolymfr formed frp, (2-acrylamido-2-methylpropane-3-sulphonic acid) and acrylic acid and/or acrylamide. Such copolymers are known as fluid loss control additives for cement slurries, as disclosed in U.S. Pat. No. 6,277,900, the disclosure of which is incorporated herein by reference.